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<span id="C-Dialect-Options"></span><div class="header">
<p>
Next: <a href="C_002b_002b-Dialect-Options.html" accesskey="n" rel="next">C++ Dialect Options</a>, Previous: <a href="Invoking-G_002b_002b.html" accesskey="p" rel="prev">Invoking G++</a>, Up: <a href="Invoking-GCC.html" accesskey="u" rel="up">Invoking GCC</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<span id="Options-Controlling-C-Dialect"></span><h3 class="section">3.4 Options Controlling C Dialect</h3>
<span id="index-dialect-options"></span>
<span id="index-language-dialect-options"></span>
<span id="index-options_002c-dialect"></span>

<p>The following options control the dialect of C (or languages derived
from C, such as C++, Objective-C and Objective-C++) that the compiler
accepts:
</p>
<dl compact="compact">
<dd><span id="index-ANSI-support"></span>
<span id="index-ISO-support"></span>
<span id="index-ansi-1"></span>
</dd>
<dt><code>-ansi</code></dt>
<dd><p>In C mode, this is equivalent to <samp>-std=c90</samp>. In C++ mode, it is
equivalent to <samp>-std=c++98</samp>.
</p>
<p>This turns off certain features of GCC that are incompatible with ISO
C90 (when compiling C code), or of standard C++ (when compiling C++ code),
such as the <code>asm</code> and <code>typeof</code> keywords, and
predefined macros such as <code>unix</code> and <code>vax</code> that identify the
type of system you are using.  It also enables the undesirable and
rarely used ISO trigraph feature.  For the C compiler,
it disables recognition of C++ style &lsquo;<samp>//</samp>&rsquo; comments as well as
the <code>inline</code> keyword.
</p>
<p>The alternate keywords <code>__asm__</code>, <code>__extension__</code>,
<code>__inline__</code> and <code>__typeof__</code> continue to work despite
<samp>-ansi</samp>.  You would not want to use them in an ISO C program, of
course, but it is useful to put them in header files that might be included
in compilations done with <samp>-ansi</samp>.  Alternate predefined macros
such as <code>__unix__</code> and <code>__vax__</code> are also available, with or
without <samp>-ansi</samp>.
</p>
<p>The <samp>-ansi</samp> option does not cause non-ISO programs to be
rejected gratuitously.  For that, <samp>-Wpedantic</samp> is required in
addition to <samp>-ansi</samp>.  See <a href="Warning-Options.html">Warning Options</a>.
</p>
<p>The macro <code>__STRICT_ANSI__</code> is predefined when the <samp>-ansi</samp>
option is used.  Some header files may notice this macro and refrain
from declaring certain functions or defining certain macros that the
ISO standard doesn&rsquo;t call for; this is to avoid interfering with any
programs that might use these names for other things.
</p>
<p>Functions that are normally built in but do not have semantics
defined by ISO C (such as <code>alloca</code> and <code>ffs</code>) are not built-in
functions when <samp>-ansi</samp> is used.  See <a href="Other-Builtins.html">Other
built-in functions provided by GCC</a>, for details of the functions
affected.
</p>
<span id="index-std-1"></span>
</dd>
<dt><code>-std=</code></dt>
<dd><p>Determine the language standard. See <a href="Standards.html">Language Standards
Supported by GCC</a>, for details of these standard versions.  This option
is currently only supported when compiling C or C++.
</p>
<p>The compiler can accept several base standards, such as &lsquo;<samp>c90</samp>&rsquo; or
&lsquo;<samp>c++98</samp>&rsquo;, and GNU dialects of those standards, such as
&lsquo;<samp>gnu90</samp>&rsquo; or &lsquo;<samp>gnu++98</samp>&rsquo;.  When a base standard is specified, the
compiler accepts all programs following that standard plus those
using GNU extensions that do not contradict it.  For example,
<samp>-std=c90</samp> turns off certain features of GCC that are
incompatible with ISO C90, such as the <code>asm</code> and <code>typeof</code>
keywords, but not other GNU extensions that do not have a meaning in
ISO C90, such as omitting the middle term of a <code>?:</code>
expression. On the other hand, when a GNU dialect of a standard is
specified, all features supported by the compiler are enabled, even when
those features change the meaning of the base standard.  As a result, some
strict-conforming programs may be rejected.  The particular standard
is used by <samp>-Wpedantic</samp> to identify which features are GNU
extensions given that version of the standard. For example
<samp>-std=gnu90 -Wpedantic</samp> warns about C++ style &lsquo;<samp>//</samp>&rsquo;
comments, while <samp>-std=gnu99 -Wpedantic</samp> does not.
</p>
<p>A value for this option must be provided; possible values are
</p>
<dl compact="compact">
<dt>&lsquo;<samp>c90</samp>&rsquo;</dt>
<dt>&lsquo;<samp>c89</samp>&rsquo;</dt>
<dt>&lsquo;<samp>iso9899:1990</samp>&rsquo;</dt>
<dd><p>Support all ISO C90 programs (certain GNU extensions that conflict
with ISO C90 are disabled). Same as <samp>-ansi</samp> for C code.
</p>
</dd>
<dt>&lsquo;<samp>iso9899:199409</samp>&rsquo;</dt>
<dd><p>ISO C90 as modified in amendment 1.
</p>
</dd>
<dt>&lsquo;<samp>c99</samp>&rsquo;</dt>
<dt>&lsquo;<samp>c9x</samp>&rsquo;</dt>
<dt>&lsquo;<samp>iso9899:1999</samp>&rsquo;</dt>
<dt>&lsquo;<samp>iso9899:199x</samp>&rsquo;</dt>
<dd><p>ISO C99.  This standard is substantially completely supported, modulo
bugs and floating-point issues
(mainly but not entirely relating to optional C99 features from
Annexes F and G).  See
<a href="https://gcc.gnu.org/c99status.html">https://gcc.gnu.org/c99status.html</a><!-- /@w --> for more information.  The
names &lsquo;<samp>c9x</samp>&rsquo; and &lsquo;<samp>iso9899:199x</samp>&rsquo; are deprecated.
</p>
</dd>
<dt>&lsquo;<samp>c11</samp>&rsquo;</dt>
<dt>&lsquo;<samp>c1x</samp>&rsquo;</dt>
<dt>&lsquo;<samp>iso9899:2011</samp>&rsquo;</dt>
<dd><p>ISO C11, the 2011 revision of the ISO C standard.  This standard is
substantially completely supported, modulo bugs, floating-point issues
(mainly but not entirely relating to optional C11 features from
Annexes F and G) and the optional Annexes K (Bounds-checking
interfaces) and L (Analyzability).  The name &lsquo;<samp>c1x</samp>&rsquo; is deprecated.
</p>
</dd>
<dt>&lsquo;<samp>c17</samp>&rsquo;</dt>
<dt>&lsquo;<samp>c18</samp>&rsquo;</dt>
<dt>&lsquo;<samp>iso9899:2017</samp>&rsquo;</dt>
<dt>&lsquo;<samp>iso9899:2018</samp>&rsquo;</dt>
<dd><p>ISO C17, the 2017 revision of the ISO C standard
(published in 2018).  This standard is
same as C11 except for corrections of defects (all of which are also
applied with <samp>-std=c11</samp>) and a new value of
<code>__STDC_VERSION__</code>, and so is supported to the same extent as C11.
</p>
</dd>
<dt>&lsquo;<samp>c2x</samp>&rsquo;</dt>
<dd><p>The next version of the ISO C standard, still under development.  The
support for this version is experimental and incomplete.
</p>
</dd>
<dt>&lsquo;<samp>gnu90</samp>&rsquo;</dt>
<dt>&lsquo;<samp>gnu89</samp>&rsquo;</dt>
<dd><p>GNU dialect of ISO C90 (including some C99 features).
</p>
</dd>
<dt>&lsquo;<samp>gnu99</samp>&rsquo;</dt>
<dt>&lsquo;<samp>gnu9x</samp>&rsquo;</dt>
<dd><p>GNU dialect of ISO C99.  The name &lsquo;<samp>gnu9x</samp>&rsquo; is deprecated.
</p>
</dd>
<dt>&lsquo;<samp>gnu11</samp>&rsquo;</dt>
<dt>&lsquo;<samp>gnu1x</samp>&rsquo;</dt>
<dd><p>GNU dialect of ISO C11.
The name &lsquo;<samp>gnu1x</samp>&rsquo; is deprecated.
</p>
</dd>
<dt>&lsquo;<samp>gnu17</samp>&rsquo;</dt>
<dt>&lsquo;<samp>gnu18</samp>&rsquo;</dt>
<dd><p>GNU dialect of ISO C17.  This is the default for C code.
</p>
</dd>
<dt>&lsquo;<samp>gnu2x</samp>&rsquo;</dt>
<dd><p>The next version of the ISO C standard, still under development, plus
GNU extensions.  The support for this version is experimental and
incomplete.
</p>
</dd>
<dt>&lsquo;<samp>c++98</samp>&rsquo;</dt>
<dt>&lsquo;<samp>c++03</samp>&rsquo;</dt>
<dd><p>The 1998 ISO C++ standard plus the 2003 technical corrigendum and some
additional defect reports. Same as <samp>-ansi</samp> for C++ code.
</p>
</dd>
<dt>&lsquo;<samp>gnu++98</samp>&rsquo;</dt>
<dt>&lsquo;<samp>gnu++03</samp>&rsquo;</dt>
<dd><p>GNU dialect of <samp>-std=c++98</samp>.
</p>
</dd>
<dt>&lsquo;<samp>c++11</samp>&rsquo;</dt>
<dt>&lsquo;<samp>c++0x</samp>&rsquo;</dt>
<dd><p>The 2011 ISO C++ standard plus amendments.
The name &lsquo;<samp>c++0x</samp>&rsquo; is deprecated.
</p>
</dd>
<dt>&lsquo;<samp>gnu++11</samp>&rsquo;</dt>
<dt>&lsquo;<samp>gnu++0x</samp>&rsquo;</dt>
<dd><p>GNU dialect of <samp>-std=c++11</samp>.
The name &lsquo;<samp>gnu++0x</samp>&rsquo; is deprecated.
</p>
</dd>
<dt>&lsquo;<samp>c++14</samp>&rsquo;</dt>
<dt>&lsquo;<samp>c++1y</samp>&rsquo;</dt>
<dd><p>The 2014 ISO C++ standard plus amendments.
The name &lsquo;<samp>c++1y</samp>&rsquo; is deprecated.
</p>
</dd>
<dt>&lsquo;<samp>gnu++14</samp>&rsquo;</dt>
<dt>&lsquo;<samp>gnu++1y</samp>&rsquo;</dt>
<dd><p>GNU dialect of <samp>-std=c++14</samp>.
The name &lsquo;<samp>gnu++1y</samp>&rsquo; is deprecated.
</p>
</dd>
<dt>&lsquo;<samp>c++17</samp>&rsquo;</dt>
<dt>&lsquo;<samp>c++1z</samp>&rsquo;</dt>
<dd><p>The 2017 ISO C++ standard plus amendments.
The name &lsquo;<samp>c++1z</samp>&rsquo; is deprecated.
</p>
</dd>
<dt>&lsquo;<samp>gnu++17</samp>&rsquo;</dt>
<dt>&lsquo;<samp>gnu++1z</samp>&rsquo;</dt>
<dd><p>GNU dialect of <samp>-std=c++17</samp>.
This is the default for C++ code.
The name &lsquo;<samp>gnu++1z</samp>&rsquo; is deprecated.
</p>
</dd>
<dt>&lsquo;<samp>c++20</samp>&rsquo;</dt>
<dt>&lsquo;<samp>c++2a</samp>&rsquo;</dt>
<dd><p>The 2020 ISO C++ standard plus amendments.
Support is experimental, and could change in incompatible ways in
future releases.
The name &lsquo;<samp>c++2a</samp>&rsquo; is deprecated.
</p>
</dd>
<dt>&lsquo;<samp>gnu++20</samp>&rsquo;</dt>
<dt>&lsquo;<samp>gnu++2a</samp>&rsquo;</dt>
<dd><p>GNU dialect of <samp>-std=c++20</samp>.
Support is experimental, and could change in incompatible ways in
future releases.
The name &lsquo;<samp>gnu++2a</samp>&rsquo; is deprecated.
</p>
</dd>
<dt>&lsquo;<samp>c++2b</samp>&rsquo;</dt>
<dt>&lsquo;<samp>c++23</samp>&rsquo;</dt>
<dd><p>The next revision of the ISO C++ standard, planned for
2023.  Support is highly experimental, and will almost certainly
change in incompatible ways in future releases.
</p>
</dd>
<dt>&lsquo;<samp>gnu++2b</samp>&rsquo;</dt>
<dt>&lsquo;<samp>gnu++23</samp>&rsquo;</dt>
<dd><p>GNU dialect of <samp>-std=c++2b</samp>.  Support is highly experimental,
and will almost certainly change in incompatible ways in future
releases.
</p></dd>
</dl>

<span id="index-aux_002dinfo"></span>
</dd>
<dt><code>-aux-info <var>filename</var></code></dt>
<dd><p>Output to the given filename prototyped declarations for all functions
declared and/or defined in a translation unit, including those in header
files.  This option is silently ignored in any language other than C.
</p>
<p>Besides declarations, the file indicates, in comments, the origin of
each declaration (source file and line), whether the declaration was
implicit, prototyped or unprototyped (&lsquo;<samp>I</samp>&rsquo;, &lsquo;<samp>N</samp>&rsquo; for new or
&lsquo;<samp>O</samp>&rsquo; for old, respectively, in the first character after the line
number and the colon), and whether it came from a declaration or a
definition (&lsquo;<samp>C</samp>&rsquo; or &lsquo;<samp>F</samp>&rsquo;, respectively, in the following
character).  In the case of function definitions, a K&amp;R-style list of
arguments followed by their declarations is also provided, inside
comments, after the declaration.
</p>
<span id="index-fno_002dasm"></span>
<span id="index-fasm"></span>
</dd>
<dt><code>-fno-asm</code></dt>
<dd><p>Do not recognize <code>asm</code>, <code>inline</code> or <code>typeof</code> as a
keyword, so that code can use these words as identifiers.  You can use
the keywords <code>__asm__</code>, <code>__inline__</code> and <code>__typeof__</code>
instead.  In C, <samp>-ansi</samp> implies <samp>-fno-asm</samp>.
</p>
<p>In C++, <code>inline</code> is a standard keyword and is not affected by
this switch.  You may want to use the <samp>-fno-gnu-keywords</samp> flag
instead, which disables <code>typeof</code> but not <code>asm</code> and
<code>inline</code>.  In C99 mode (<samp>-std=c99</samp> or <samp>-std=gnu99</samp>),
this switch only affects the <code>asm</code> and <code>typeof</code> keywords,
since <code>inline</code> is a standard keyword in ISO C99.  In C2X mode
(<samp>-std=c2x</samp> or <samp>-std=gnu2x</samp>), this switch only affects
the <code>asm</code> keyword, since <code>typeof</code> is a standard keyword in
ISO C2X.
</p>
<span id="index-fno_002dbuiltin"></span>
<span id="index-fbuiltin"></span>
<span id="index-built_002din-functions"></span>
</dd>
<dt><code>-fno-builtin</code></dt>
<dt><code>-fno-builtin-<var>function</var></code></dt>
<dd><p>Don&rsquo;t recognize built-in functions that do not begin with
&lsquo;<samp>__builtin_</samp>&rsquo; as prefix.  See <a href="Other-Builtins.html">Other built-in
functions provided by GCC</a>, for details of the functions affected,
including those which are not built-in functions when <samp>-ansi</samp> or
<samp>-std</samp> options for strict ISO C conformance are used because they
do not have an ISO standard meaning.
</p>
<p>GCC normally generates special code to handle certain built-in functions
more efficiently; for instance, calls to <code>alloca</code> may become single
instructions which adjust the stack directly, and calls to <code>memcpy</code>
may become inline copy loops.  The resulting code is often both smaller
and faster, but since the function calls no longer appear as such, you
cannot set a breakpoint on those calls, nor can you change the behavior
of the functions by linking with a different library.  In addition,
when a function is recognized as a built-in function, GCC may use
information about that function to warn about problems with calls to
that function, or to generate more efficient code, even if the
resulting code still contains calls to that function.  For example,
warnings are given with <samp>-Wformat</samp> for bad calls to
<code>printf</code> when <code>printf</code> is built in and <code>strlen</code> is
known not to modify global memory.
</p>
<p>With the <samp>-fno-builtin-<var>function</var></samp> option
only the built-in function <var>function</var> is
disabled.  <var>function</var> must not begin with &lsquo;<samp>__builtin_</samp>&rsquo;.  If a
function is named that is not built-in in this version of GCC, this
option is ignored.  There is no corresponding
<samp>-fbuiltin-<var>function</var></samp> option; if you wish to enable
built-in functions selectively when using <samp>-fno-builtin</samp> or
<samp>-ffreestanding</samp>, you may define macros such as:
</p>
<div class="example">
<pre class="example">#define abs(n)          __builtin_abs ((n))
#define strcpy(d, s)    __builtin_strcpy ((d), (s))
</pre></div>

<span id="index-fcond_002dmismatch"></span>
</dd>
<dt><code>-fcond-mismatch</code></dt>
<dd><p>Allow conditional expressions with mismatched types in the second and
third arguments.  The value of such an expression is void.  This option
is not supported for C++.
</p>
<span id="index-ffreestanding-1"></span>
<span id="index-hosted-environment-1"></span>
</dd>
<dt><code>-ffreestanding</code></dt>
<dd>
<p>Assert that compilation targets a freestanding environment.  This
implies <samp>-fno-builtin</samp>.  A freestanding environment
is one in which the standard library may not exist, and program startup may
not necessarily be at <code>main</code>.  The most obvious example is an OS kernel.
This is equivalent to <samp>-fno-hosted</samp>.
</p>
<p>See <a href="Standards.html">Language Standards Supported by GCC</a>, for details of
freestanding and hosted environments.
</p>
<span id="index-fgimple"></span>
</dd>
<dt><code>-fgimple</code></dt>
<dd>
<p>Enable parsing of function definitions marked with <code>__GIMPLE</code>.
This is an experimental feature that allows unit testing of GIMPLE
passes.
</p>
<span id="index-fgnu_002dtm"></span>
</dd>
<dt><code>-fgnu-tm</code></dt>
<dd><p>When the option <samp>-fgnu-tm</samp> is specified, the compiler
generates code for the Linux variant of Intel&rsquo;s current Transactional
Memory ABI specification document (Revision 1.1, May 6 2009).  This is
an experimental feature whose interface may change in future versions
of GCC, as the official specification changes.  Please note that not
all architectures are supported for this feature.
</p>
<p>For more information on GCC&rsquo;s support for transactional memory,
See <a href="../libitm/Enabling-libitm.html#Enabling-libitm">The GNU Transactional Memory Library</a> in <cite>GNU
Transactional Memory Library</cite>.
</p>
<p>Note that the transactional memory feature is not supported with
non-call exceptions (<samp>-fnon-call-exceptions</samp>).
</p>
<span id="index-fgnu89_002dinline"></span>
</dd>
<dt><code>-fgnu89-inline</code></dt>
<dd><p>The option <samp>-fgnu89-inline</samp> tells GCC to use the traditional
GNU semantics for <code>inline</code> functions when in C99 mode.
See <a href="Inline.html">An Inline Function is As Fast As a Macro</a>.
Using this option is roughly equivalent to adding the
<code>gnu_inline</code> function attribute to all inline functions
(see <a href="Function-Attributes.html">Function Attributes</a>).
</p>
<p>The option <samp>-fno-gnu89-inline</samp> explicitly tells GCC to use the
C99 semantics for <code>inline</code> when in C99 or gnu99 mode (i.e., it
specifies the default behavior).
This option is not supported in <samp>-std=c90</samp> or
<samp>-std=gnu90</samp> mode.
</p>
<p>The preprocessor macros <code>__GNUC_GNU_INLINE__</code> and
<code>__GNUC_STDC_INLINE__</code> may be used to check which semantics are
in effect for <code>inline</code> functions.  See <a href="https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html#Common-Predefined-Macros">Common Predefined
Macros</a> in <cite>The C Preprocessor</cite>.
</p>
<span id="index-fhosted"></span>
<span id="index-hosted-environment-2"></span>
</dd>
<dt><code>-fhosted</code></dt>
<dd>
<p>Assert that compilation targets a hosted environment.  This implies
<samp>-fbuiltin</samp>.  A hosted environment is one in which the
entire standard library is available, and in which <code>main</code> has a return
type of <code>int</code>.  Examples are nearly everything except a kernel.
This is equivalent to <samp>-fno-freestanding</samp>.
</p>
<span id="index-flax_002dvector_002dconversions"></span>
</dd>
<dt><code>-flax-vector-conversions</code></dt>
<dd><p>Allow implicit conversions between vectors with differing numbers of
elements and/or incompatible element types.  This option should not be
used for new code.
</p>
<span id="index-fms_002dextensions"></span>
</dd>
<dt><code>-fms-extensions</code></dt>
<dd><p>Accept some non-standard constructs used in Microsoft header files.
</p>
<p>In C++ code, this allows member names in structures to be similar
to previous types declarations.
</p>
<div class="example">
<pre class="example">typedef int UOW;
struct ABC {
  UOW UOW;
};
</pre></div>

<p>Some cases of unnamed fields in structures and unions are only
accepted with this option.  See <a href="Unnamed-Fields.html">Unnamed struct/union
fields within structs/unions</a>, for details.
</p>
<p>Note that this option is off for all targets except for x86
targets using ms-abi.
</p>
<span id="index-foffload"></span>
<span id="index-Offloading-targets"></span>
<span id="index-OpenACC-offloading-targets"></span>
<span id="index-OpenMP-offloading-targets"></span>
</dd>
<dt><code>-foffload=disable</code></dt>
<dt><code>-foffload=default</code></dt>
<dt><code>-foffload=<var>target-list</var></code></dt>
<dd><p>Specify for which OpenMP and OpenACC offload targets code should be generated.
The default behavior, equivalent to <samp>-foffload=default</samp>, is to generate
code for all supported offload targets.  The <samp>-foffload=disable</samp> form
generates code only for the host fallback, while
<samp>-foffload=<var>target-list</var></samp> generates code only for the specified
comma-separated list of offload targets.
</p>
<p>Offload targets are specified in GCC&rsquo;s internal target-triplet format. You can
run the compiler with <samp>-v</samp> to show the list of configured offload targets
under <code>OFFLOAD_TARGET_NAMES</code>.
</p>
<span id="index-foffload_002doptions"></span>
<span id="index-Offloading-options"></span>
<span id="index-OpenACC-offloading-options"></span>
<span id="index-OpenMP-offloading-options"></span>
</dd>
<dt><code>-foffload-options=<var>options</var></code></dt>
<dt><code>-foffload-options=<var>target-triplet-list</var>=<var>options</var></code></dt>
<dd>
<p>With <samp>-foffload-options=<var>options</var></samp>, GCC passes the specified
<var>options</var> to the compilers for all enabled offloading targets.  You can
specify options that apply only to a specific target or targets by using
the <samp>-foffload-options=<var>target-list</var>=<var>options</var></samp> form.  The
<var>target-list</var> is a comma-separated list in the same format as for the
<samp>-foffload=</samp> option.
</p>
<p>Typical command lines are
</p>
<div class="example">
<pre class="example">-foffload-options=-lgfortran -foffload-options=-lm
-foffload-options=&quot;-lgfortran -lm&quot; -foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906 -foffload-options=-lm
</pre></div>

<span id="index-fopenacc"></span>
<span id="index-OpenACC-accelerator-programming"></span>
</dd>
<dt><code>-fopenacc</code></dt>
<dd><p>Enable handling of OpenACC directives <code>#pragma acc</code> in C/C++ and
<code>!$acc</code> in Fortran.  When <samp>-fopenacc</samp> is specified, the
compiler generates accelerated code according to the OpenACC Application
Programming Interface v2.6 <a href="https://www.openacc.org">https://www.openacc.org</a><!-- /@w -->.  This option
implies <samp>-pthread</samp>, and thus is only supported on targets that
have support for <samp>-pthread</samp>.
</p>
<span id="index-fopenacc_002ddim"></span>
<span id="index-OpenACC-accelerator-programming-1"></span>
</dd>
<dt><code>-fopenacc-dim=<var>geom</var></code></dt>
<dd><p>Specify default compute dimensions for parallel offload regions that do
not explicitly specify.  The <var>geom</var> value is a triple of
&rsquo;:&rsquo;-separated sizes, in order &rsquo;gang&rsquo;, &rsquo;worker&rsquo; and, &rsquo;vector&rsquo;.  A size
can be omitted, to use a target-specific default value.
</p>
<span id="index-fopenmp"></span>
<span id="index-OpenMP-parallel"></span>
</dd>
<dt><code>-fopenmp</code></dt>
<dd><p>Enable handling of OpenMP directives <code>#pragma omp</code> in C/C++,
<code>[[omp::directive(...)]]</code> and <code>[[omp::sequence(...)]]</code> in C++ and
<code>!$omp</code> in Fortran.  When <samp>-fopenmp</samp> is specified, the
compiler generates parallel code according to the OpenMP Application
Program Interface v4.5 <a href="https://www.openmp.org">https://www.openmp.org</a><!-- /@w -->.  This option
implies <samp>-pthread</samp>, and thus is only supported on targets that
have support for <samp>-pthread</samp>. <samp>-fopenmp</samp> implies
<samp>-fopenmp-simd</samp>.
</p>
<span id="index-fopenmp_002dsimd"></span>
<span id="index-OpenMP-SIMD"></span>
<span id="index-SIMD"></span>
</dd>
<dt><code>-fopenmp-simd</code></dt>
<dd><p>Enable handling of OpenMP&rsquo;s <code>simd</code>, <code>declare simd</code>,
<code>declare reduction</code>, <code>assume</code>, <code>ordered</code>, <code>scan</code>,
<code>loop</code> directives and combined or composite directives with
<code>simd</code> as constituent with <code>#pragma omp</code> in C/C++,
<code>[[omp::directive(...)]]</code> and <code>[[omp::sequence(...)]]</code> in C++
and <code>!$omp</code> in Fortran.  Other OpenMP directives are ignored.
</p>
<span id="index-fopenmp_002dtarget_002dsimd_002dclone"></span>
<span id="index-OpenMP-target-SIMD-clone"></span>
</dd>
<dt><code>-fopenmp-target-simd-clone</code></dt>
<dt><code>-fopenmp-target-simd-clone=<var>device-type</var></code></dt>
<dd><p>In addition to generating SIMD clones for functions marked with the
<code>declare simd</code> directive, GCC also generates clones
for functions marked with the OpenMP <code>declare target</code> directive
that are suitable for vectorization when this option is in effect.  The
<var>device-type</var> may be one of <code>none</code>, <code>host</code>, <code>nohost</code>,
and <code>any</code>, which correspond to keywords for the <code>device_type</code>
clause of the <code>declare target</code> directive; clones are generated for
the intersection of devices specified.
<samp>-fopenmp-target-simd-clone</samp> is equivalent to
<samp>-fopenmp-target-simd-clone=any</samp> and
<samp>-fno-openmp-target-simd-clone</samp> is equivalent to
<samp>-fopenmp-target-simd-clone=none</samp>.
</p>
<p>At <samp>-O2</samp> and higher (but not <samp>-Os</samp> or <samp>-Og</samp>) this
optimization defaults to <samp>-fopenmp-target-simd-clone=nohost</samp>; otherwise
it is disabled by default.
</p>
<span id="index-fpermitted_002dflt_002deval_002dmethods"></span>
<span id="index-fpermitted_002dflt_002deval_002dmethods_003dc11"></span>
<span id="index-fpermitted_002dflt_002deval_002dmethods_003dts_002d18661_002d3"></span>
</dd>
<dt><code>-fpermitted-flt-eval-methods=<var>style</var></code></dt>
<dd><p>ISO/IEC TS 18661-3 defines new permissible values for
<code>FLT_EVAL_METHOD</code> that indicate that operations and constants with
a semantic type that is an interchange or extended format should be
evaluated to the precision and range of that type.  These new values are
a superset of those permitted under C99/C11, which does not specify the
meaning of other positive values of <code>FLT_EVAL_METHOD</code>.  As such, code
conforming to C11 may not have been written expecting the possibility of
the new values.
</p>
<p><samp>-fpermitted-flt-eval-methods</samp> specifies whether the compiler
should allow only the values of <code>FLT_EVAL_METHOD</code> specified in C99/C11,
or the extended set of values specified in ISO/IEC TS 18661-3.
</p>
<p><var>style</var> is either <code>c11</code> or <code>ts-18661-3</code> as appropriate.
</p>
<p>The default when in a standards compliant mode (<samp>-std=c11</samp> or similar)
is <samp>-fpermitted-flt-eval-methods=c11</samp>.  The default when in a GNU
dialect (<samp>-std=gnu11</samp> or similar) is
<samp>-fpermitted-flt-eval-methods=ts-18661-3</samp>.
</p>
<span id="index-fplan9_002dextensions"></span>
</dd>
<dt><code>-fplan9-extensions</code></dt>
<dd><p>Accept some non-standard constructs used in Plan 9 code.
</p>
<p>This enables <samp>-fms-extensions</samp>, permits passing pointers to
structures with anonymous fields to functions that expect pointers to
elements of the type of the field, and permits referring to anonymous
fields declared using a typedef.  See <a href="Unnamed-Fields.html">Unnamed
struct/union fields within structs/unions</a>, for details.  This is only
supported for C, not C++.
</p>
<span id="index-fsigned_002dbitfields"></span>
<span id="index-funsigned_002dbitfields"></span>
<span id="index-fno_002dsigned_002dbitfields"></span>
<span id="index-fno_002dunsigned_002dbitfields"></span>
</dd>
<dt><code>-fsigned-bitfields</code></dt>
<dt><code>-funsigned-bitfields</code></dt>
<dt><code>-fno-signed-bitfields</code></dt>
<dt><code>-fno-unsigned-bitfields</code></dt>
<dd><p>These options control whether a bit-field is signed or unsigned, when the
declaration does not use either <code>signed</code> or <code>unsigned</code>.  By
default, such a bit-field is signed, because this is consistent: the
basic integer types such as <code>int</code> are signed types.
</p>
<span id="index-fsigned_002dchar"></span>
</dd>
<dt><code>-fsigned-char</code></dt>
<dd><p>Let the type <code>char</code> be signed, like <code>signed char</code>.
</p>
<p>Note that this is equivalent to <samp>-fno-unsigned-char</samp>, which is
the negative form of <samp>-funsigned-char</samp>.  Likewise, the option
<samp>-fno-signed-char</samp> is equivalent to <samp>-funsigned-char</samp>.
</p>
<span id="index-funsigned_002dchar"></span>
</dd>
<dt><code>-funsigned-char</code></dt>
<dd><p>Let the type <code>char</code> be unsigned, like <code>unsigned char</code>.
</p>
<p>Each kind of machine has a default for what <code>char</code> should
be.  It is either like <code>unsigned char</code> by default or like
<code>signed char</code> by default.
</p>
<p>Ideally, a portable program should always use <code>signed char</code> or
<code>unsigned char</code> when it depends on the signedness of an object.
But many programs have been written to use plain <code>char</code> and
expect it to be signed, or expect it to be unsigned, depending on the
machines they were written for.  This option, and its inverse, let you
make such a program work with the opposite default.
</p>
<p>The type <code>char</code> is always a distinct type from each of
<code>signed char</code> or <code>unsigned char</code>, even though its behavior
is always just like one of those two.
</p>
<span id="index-fstrict_002dflex_002darrays"></span>
<span id="index-fno_002dstrict_002dflex_002darrays"></span>
</dd>
<dt><code>-fstrict-flex-arrays</code></dt>
<dd><p>Control when to treat the trailing array of a structure as a flexible array
member for the purpose of accessing the elements of such an array.
The positive form is equivalent to <samp>-fstrict-flex-arrays=3</samp>, which is the
strictest.  A trailing array is treated as a flexible array member only when it
is declared as a flexible array member per C99 standard onwards.
The negative form is equivalent to <samp>-fstrict-flex-arrays=0</samp>, which is the
least strict.  All trailing arrays of structures are treated as flexible array
members.
</p>
<span id="index-fstrict_002dflex_002darrays_003dlevel"></span>
</dd>
<dt><code>-fstrict-flex-arrays=<var>level</var></code></dt>
<dd><p>Control when to treat the trailing array of a structure as a flexible array
member for the purpose of accessing the elements of such an array.  The value
of <var>level</var> controls the level of strictness.
</p>
<p>The possible values of <var>level</var> are the same as for the
<code>strict_flex_array</code> attribute (see <a href="Variable-Attributes.html">Variable Attributes</a>).
</p>
<p>You can control this behavior for a specific trailing array field of a
structure by using the variable attribute <code>strict_flex_array</code> attribute
(see <a href="Variable-Attributes.html">Variable Attributes</a>).
</p>
<span id="index-fsso_002dstruct"></span>
</dd>
<dt><code>-fsso-struct=<var>endianness</var></code></dt>
<dd><p>Set the default scalar storage order of structures and unions to the
specified endianness.  The accepted values are &lsquo;<samp>big-endian</samp>&rsquo;,
&lsquo;<samp>little-endian</samp>&rsquo; and &lsquo;<samp>native</samp>&rsquo; for the native endianness of
the target (the default).  This option is not supported for C++.
</p>
<p><strong>Warning:</strong> the <samp>-fsso-struct</samp> switch causes GCC to generate
code that is not binary compatible with code generated without it if the
specified endianness is not the native endianness of the target.
</p></dd>
</dl>

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